Never trust pet store employees when it comes to fish, especially betta fish. You need water conditioner for any water you're going to put the fish in. There are certain metals in the water we drink that, while not harmful to us, can easily poison and kill our fish. A good water conditioner will get rid of these metals.
A heater is a needed thing for a betta, especially since the nights are getting cooler. Fish don't like their temperature swinging more than a degree or two during the day and night, it will stress them and make them sick. However, it's not really safe to use a heater in a tank smaller than about 2-3 gallons.
Pet store employees are taught all wrong about betta fish. Unless you happen across an employee that did their own research, they're completely useless when it comes to advice. I'd suggest running around the Internet for awhile and reading some detailed articles on caring for your little fishies. :)
Last edited by Yuuki; 10-08-2009 at 07:31 PM.